I assume you have the book by Robert Chambers? I just got it but didn’t read it yet 
There’s no grimoires I’m aware on as he’s not a “demon” from a traditional grimoiric current, anything you find will be modern - BUT then again if you are calling Hastur using this egregore as a sort of mark, then you’re free to use .
Technically I agree with DarkestKnight that he is not “Lovecraftian” as the main work is not by Lovecraft - he was invented by Chambers and Lovecraft made a mention of him. Gothic horror, yes.
There’s some great expositions about the character on youtube.
Like all fictional characters this is an egregore, which gives you a bit of leeway in inventing how he is - he’s created by humans and belongs to humans, how you work with him will have an effect on him, more strongly than with sovereign entities. But there’s a huge fanbase of fan-authored stories as well that feed into this, and it’s a gothic-horror fanbase, and humans are great at making up really awful scenarios (think Grimdark) so, yeah, be careful what you wish for, you don’t know what they have contributed into the shared astral where this entity lives.